1. Field of the Invention
The present invention relates to a method for forming a thin film of a composite metal compound and an apparatus for carrying out the method. More particularly, the invention relates to a method for forming through sputtering a thin film of a composite metal compound on a substrate in a stable manner and at a high rate of deposition and to an apparatus for carrying out the method.
2. Description of the Related Art
Conventionally, when optical thin films for various groups of products are formed through use of only existing vapor deposition materials, satisfactory performances as required by the products are very difficult to obtain. That is, designing optical thin films through use of mere substances existing in the natural world has proved difficult in terms of attaining optical spectral characteristics as required by a certain group of products.
For example, configuration of wide-band antireflection films requires materials having an intermediate refractive index (between 1.46 and 2.20), which materials rarely exist in the natural world.
Generally, in order to decrease the reflectance of, for example, glass, over the entire wavelength range of visible light, glass must be coated with a vapor deposition material having a refractive index of 1.46-2.20, called an intermediate refractive index. Materials having an intermediate refractive index are limited, and the refractive index cannot be selected as desired. Accordingly, the following techniques are known as alternative techniques for obtaining an intermediate refractive index in the above-mentioned range.
(1) A low-refraction material (e.g. SiO.sub.2 (refractive index: 1.46)) and a high-refraction material (e.g. TiO.sub.2 (refractive index: 2.35)) are concurrently evaporated from respective evaporation sources, and an intermediate refractive index (1.46-2.40) is obtained by virtue of their mixing ratio; (2) a low-refraction material and a high-refraction material are concurrently evaporated from a single evaporation source in the form of a mixture, and an intermediate refractive index (1.46-2.40) is obtained by virtue of their mixing ratio; (3) an intermediate refractive index is equivalently obtained through the combination of a low-refraction material and a high-refraction material (called the equivalent film technique); and (4) a composite target material is used in sputtering.
However, the above-mentioned techniques involve the following disadvantages.
In the above-mentioned technique (1), wherein a low-refraction material (e.g. SiO.sub.2 (refractive index: 1.46)) and a high-refraction material (e.g. TiO.sub.2 ((refractive index: 2.35)) are concurrently evaporated from respective evaporation sources and an intermediate refractive index (1.46-2.40) is obtained by virtue of their mixing ratio, the stable deposition of a film through the simultaneous control of the rates of deposition from the two evaporation sources is difficult to achieve, and thus a desired refractive index is difficult to obtain with good reproducibility.
In the above-mentioned technique (2), wherein a low-refraction material and a high-refraction material are concurrently evaporated from a single evaporation source in the form of a mixture and an intermediate refractive index is obtained by virtue of their mixing ratio, when evaporation continues for a long period of time, the refractive index changes due to differences in melting point and vapor pressure between the low-refraction material and the high-refraction material. As a result, a desired refractive index is difficult to obtain stably.
In the above-mentioned technique (3), wherein an intermediate refractive index (1.46-2.40) is obtained through use of an equivalent film formed from combined use of low-refraction and high-refraction materials, a given refractive index requires the formation of a very thin layer; thus, the control of film thickness becomes difficult and complicated.
As mentioned above, the conventional techniques fail to concurrently implement a high, stable deposition rate, a wide range of refractive index variation, and a simple control system.